1. Field of the Invention
This invention relates to an improved method of oxidizing halogenated hydrocarbons. More particularly, this method provides for pretreating the halogenated hydrocarbon feedstream prior to its introduction into a hydrothermal oxidation reactor. The pretreatment comprises adding an alkali and water at hydrothermal conditions, which result in hydrolysis of the halogenated hydrocarbons and neutralization of the liberated halogen ions.
2. Description of the Related Art
The process of "wet oxidation" has been used for the oxidation of compounds in an aqueous stream for some time. Generally, it involves the addition of an oxidizing agent, typically air or oxygen, to an aqueous stream at elevated temperatures and pressures, with the resultant "combustion" of oxidizable materials directly within the aqueous phase. This wet oxidation process is characterized by operating pressures of 25 to 250 bar (440 to 3630 psia) and operating temperatures of 150.degree. to 370.degree. C. At these conditions, the gas phase oxidation is quite slow and the majority of the oxidation reaction is carried out in the liquid phase. Thus, the reactor operating conditions are typically maintained at or about the saturation point of water, such that at least a part of the water is present in liquid form. This wet oxidation process has several drawbacks. First, it is unsuitable to adequately handle refractory compounds. Second, it is characterized by slow reaction times. Third, due to the low temperature of the process, heat recovery is limited.
In light of such limitations, aqueous oxidation processes were extended to higher temperatures and pressures. In U.S. Pat. No. 2,944,396 to Barton et. al., the addition of a second oxidation stage after a wet oxidation reactor is taught. Unoxidized volatile combustibles which accumulate in the vapor phase of the first stage wet oxidation reactor are oxidized in a second stage, which is operated at temperatures above the critical temperature of water of about 374.degree. C. U.S. Pat. No. 4,292,953 to Dickinson, discloses a modified wet oxidation process for power generation from coal and other fuels in which, as heat is liberated by combustion, the reaction mixture exceeds the critical temperature of water, with operating pressures of about 69 bar (1000 psi) to about 690 bar (10,000 psi) spanning both the sub-- and supercritical water pressure ranges. U.S. Pat. No. 4,338,199 to Modell, discloses a wet oxidation process which has come to be known as supercritical water oxidation (SCWO) because in some implementations oxidation occurs essentially entirely at conditions supercritical in temperature (&gt;374.degree. C.) and pressure (&gt;about 3200 psi or 220 bar). SCWO at 500.degree.-600.degree. C. and 250 bar has been shown to give rapid and near complete oxidation of organic compounds. A related process known as supercritical temperature water oxidation (STWO) can provide similar oxidation effectiveness for certain feedstocks but at lower pressure. This process has been described in U.S. Pat. No. 5,106,513 to Hong, and utilizes temperatures in the range of 600.degree. C. and pressures between 25 and 220 bar.
These aqueous oxidation processes achieving substantially complete oxidation will hereinafter be referred to collectively as "hydrothermal oxidation" if carried out at a temperature above about 374.degree. C. and pressures above about 25 bar.
Water at supercritical temperatures and elevated pressures can be useful in carrying out many reactions with organic materials other than complete oxidation. For example, U.S. Pat. No. 2,864,677 to Eastman; U.S. Pat. No. 3,743,606 to Marlon; U.S. Pat. No. 3,607,157 to Schlinger; and, U.S. Pat. No. 4,113,446 to Modell.
The above mentioned hydrothermal oxidation suggests comparison to incineration. Carbon and hydrogen form the conventional combustion products CO.sub.2 and H.sub.2 O. Halogenated hydrocarbons may form strong acids, for example, chlorinated hydrocarbons (CHCs) may give rise to HCl. The formation of strong halogen acids may lead to acid corrosion problems for the processing equipment. In the past, alkali has been added to mitigate acid corrosion problems. The alkali neutralizes the halogen acid and forms a salt. This addition of alkali has, however, caused problems with the precipitating out, as a solid, of the salt formed upon neutralization and the subsequent build-up of the salt in the reactor and downstream equipment, or problems with poor neutralization efficiency due to the interaction of the alkali, e.g., caustic, with the carbon dioxide formed in the oxidation process.
Several patents have recognized that, in the aqueous oxidation of organic compounds, if contaminants or halogen components are present, it may be advantageous to add alkali to the oxidation reactor to neutralize the contaminant or halogen component. U.S. Pat. No. 4,714,032 to Dickinson employs the addition of alkali to the oxidation reactor to neutralize sulfur present in the carbonaceous fuel feed. U.S. Pat. No. 4,543,190 to Modell shows the use of supercritical water to dechlorinate trichloroethylene. The HCl formed is subsequently neutralized with sodium hydroxide and the mixture is then directed to the reactor to be oxidized. This procedure, however, forms high acid conditions and exposes the process equipment to hot acidic environments. Furthermore, these patents do not provide satisfactory solutions to the problems mentioned above, namely build-up of solid precipitated salt on the oxidation reactor and downstream equipment, high corrosion due to acid content and poor neutralization efficiency.
For feedstocks which contain significant amounts of organic halogens, particularly organic chlorine, the aqueous oxidation process can be constrained by conflicting requirements. Use of alkali to mitigate acid corrosion problems can exacerbate the problems of salt deposition. Also, for certain oxidation reactor designs, the feed introduction methods which maximize the degree of neutralization can worsen the salt deposition problem, and feed introduction methods which minimize deposition can result in poor neutralization efficiency.
Thus, there exists a need for a method of pretreating halogenated hydrocarbon feedstreams prior to introducing the feedstreams into an oxidation reactor, whereby acid corrosion and salt precipitation problems are mitigated, and which provides high neutralization efficiencies.